1. Field of the Invention
The present invention relates to a technique of coding and decoding moving pictures, and more particularly, to a technique of coding and decoding moving pictures using motion-compensated prediction.
2. Description of the Related Art
The MPEG-4 AVC/H.264 standard is a representative moving picture compression coding scheme. The MPEG-4 AVC/H.264 standard uses motion compensation in which a picture is partitioned into a plurality of rectangular blocks, pictures which have been coded or decoded are used as reference pictures, and a motion is predicted from the reference pictures. A method of predicting a motion based on this motion compensation is referred to as inter-prediction or motion-compensated prediction. In the inter-prediction of the MPEG-4 AVC/H.264 standard, motion compensation is performed in such a manner that a plurality of pictures can be used as reference pictures, and a most probable reference picture is selected in respective blocks from the plurality of reference pictures. Thus, a reference index is allocated to respective reference pictures and the reference pictures are specified by the reference index. In B pictures, two reference pictures at most can be selected from coded or decoded reference pictures and be used for inter-prediction. Prediction from these two reference pictures is classified into L0 prediction (list-0 prediction) which is mainly used as forward prediction and L1 prediction (list-1 prediction) which is mainly used as backward prediction.
Further, bi-prediction which uses two inter-prediction modes of L0 prediction and L1 prediction simultaneously is also defined. In the case of bi-prediction, bidirectional prediction is performed to obtain inter-prediction signals in L0 and L1 prediction modes, which are multiplied by a weighting factor and are superimposed by adding an offset value to thereby construct a final inter-prediction picture signal. As the weighting factor and offset values used for weighted prediction, a representative value for each reference picture of lists is set and coded in respective pictures. Coding information related to inter-prediction includes a prediction mode for classifying L0 prediction, L1 prediction, and bi-prediction for each block, a reference index for specifying a reference picture for each reference list of each block, and a motion vector representing a moving direction and a movement amount of a block. These items of coding information are coded or decoded.
Further, in the MPEG-4 AVC/H.264 scheme, a direct mode of constructing inter-prediction information of coding/decoding target block from inter-prediction information of a coded/decoded block is defined. Since the direct mode does not require coding of inter-prediction information, coding efficiency is improved.
A temporal direct mode which uses correlation of inter-prediction information in the temporal direction will be described with reference to FIG. 36. A picture of which the L1 reference index is registered to “0” is referred to as a reference picture colPic. A block at the same position as a coding/decoding target block in the reference picture colPic is referred to as a reference block.
If a reference block is coded using L0 prediction, a L0 motion vector of the reference block is referred to as a reference motion vector mvCol. If the reference block is not coded using L0 prediction but is coded using L1 prediction, a L1 motion vector of the reference block is referred to as a reference motion vector mvCol. A picture that the reference motion vector mvCol refers to is referred to as a L0 reference picture of the temporal direct mode, and the reference picture colPic is referred to as a L1 reference picture of the temporal direct mode.
A L0 motion vector mvL0 and a L1 motion vector mvL1 of the temporal direct mode are derived from the reference motion vector mvCol by performing a scaling process.
A POC of a L0 reference picture of the temporal direct mode is subtracted from a POC of the reference picture colPic to derive a picture-to-picture distance td. A POC is a variable associated with a picture to be coded and a value that is incremented by 1 in the picture outputting/displaying order is set as the POC. A difference between the POCs of two pictures represents a picture-to-picture distance in a time axis direction.td=(POC of reference picture colPic)−(POC of L0 reference picture of temporal direct mode)
The POC of a L0 reference picture of the temporal direct mode is subtracted from the POC of a coding/decoding target picture to derive a picture-to-picture distance tb.tb=(POC of coding/decoding target picture)−(POC of L0 reference picture of temporal direct mode)
A L0 motion vector mvL0 of the temporal direct mode is derived from the reference motion vector mvCol by performing a scaling process.mvL0=tb/td*mvCol
A reference motion vector mvCol is subtracted from the L0 motion vector mvL0 of the temporal direct mode to derive a L1 motion vector mvL1.mvL1=mvL0−mvCol
When a moving picture coding device and a moving picture decoding device have low processing capability, the process of the temporal direct mode may be omitted.    Patent Literature 1: JP 2004-129191 A
Under such a situation, the present inventors have recognized the need to further compress coding information and reduce an entire coding amount in a moving picture coding scheme which uses motion-compensated prediction.